The Mystery of the Missing Methane

Hugh Pickens writes "Astrobiology Magazine reports that NASA's Spitzer Space Telescope has discovered something odd about GJ 436b, a planet about the size of Neptune located 33 light-years away, circling the star Gliese 436. The mystery? GJ 436b lacks methane, an ingredient common to many of the planets in our solar system. Methane is present on our life-bearing planet, manufactured primarily by microbes living in cows, and all of the giant planets in our solar system have methane too, despite their lack of cows. Spitzer was able to detect the faint glow of GJ 436b by watching it slip behind its star, an event called a secondary eclipse. As the planet disappears, the total light observed from the star system drops, and the diference is then measured to find the brightness of the planet at various wavelengths. Eventually, a larger space telescope could use the same kind of technique to search smaller, Earth-like worlds for methane and other chemical signs of life, such as water, oxygen and carbon dioxide. Adam Showman, a planetary scientist at the University of Arizona, says the 'provocative result' raises questions about the evolution of this planet, as well as the possibility that its atmosphere might represent an entirely new class of atmospheres that has never been explored."

Comments

Obvious solution

[maxwell demon]

Methane is an important source of energy. Obviously there's an alien species which has used up all the methane from that planet.

Re:Obvious solution

[reverseengineer]

The reaction being looked at here is interesting because it is the same reaction used on Earth in the steam reforming of methane to produce hydrogen, with the same equilibrium issues. Methane (or many other hydrocarbons) can be reacted with water vapor to produce carbon monoxide and hydrogen. However, this reaction is not going to proceed forward under normal atmospheric conditions on Earth, and at least was not expected to proceed forward under the conditions of GJ 436b. The reaction needs enough energy put in to break apart methane and water molecules before their components can be recombined to form CO and hydrogen. In the absence of catalysts, you should expect this step to occur at temperatures no lower than around 920K, while GJ 436b is believed to be at 800K.

We can look at some of the possibilities of what could be happening on GJ 436b:
CH4 + H2O is in equilibrium with CO + 3 H2 (with a change in enthalpy of +206kJ/mol)

1. The temperature of GJ 436b could be higher than what is measured. If the temperature is actually above around 920K, then the necessary activation energy is present to get this reaction headed to the right side of the equation. This solves the mystery, but then opens a new mystery of why the temperature measurement is off by over 100K.
2. A reaction product is rapidly being taken away after formation. If either carbon monoxide or hydrogen were somehow continuously removed from the site of the reaction, the reaction equilibrium would keep favoring the generation of more CO and H2 rather than reversing to make more methane and water. This is what the suggestion of "vertical mixing" is alluding to: if the "steam methane reforming" reaction is isolated to one region of the atmosphere, but the reaction products rapidly migrate to another, then the reaction equilibrium makes sense.
3. When steam reforming of methane is done as an industrial process on Earth, the reactions are carried out at temperatures of about 700-800K, right around the temperature of GJ 436b. The necessary activation energy is lowered by metal catalysts (usually nickel) Could the interaction of the atmosphere with the rocky core be catalyzing this reaction? It's unlikely that there's enough surface area to transform the whole atmosphere in this manner, but it's an intriguing possibility.

Re:Why So Much Focus on Cows?

[maxwell demon]

Why are cows focused on so much when it comes to methane?

Because if they focused on rice, it wouldn't give a good argument for vegetarians. :-)

Re:Why So Much Focus on Cows?

[sznupi]

Cows eating grass, outside...where have you seen that? O_o

In seriousness, that's not a showstopper; especially if reduced availability of meat would, for many people, actually increase their health (I don't advocate not eating meat, I do it myself; but too many people consume ridiculous amounts of it these days, having fallen in the trap of one old adaptation - "if there's some meat around, eat it!")

This must be the mythical Planet of the Amazons

[fizzup]

This planet has got to be populated by nothing but women. Nobody farts!